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BQ40Z80: DSG pin/mosfet flickering

Karl
Prodigy 20 points
Part Number: BQ40Z80

Hi, I have a problem with my board with a BQ40Z80 on it. I am in the testing phase of the product and I and I'm trying to test the effect of a high current on my board. I attached a load of 1Ohm on PACK+ and I'm supplying the circuit with a power supply and a simulated battery pack (6x1k in series). since the load is 1 Ohm power supply voltage is mostly the same as the current trough the circuit.

When I enable DSG FET trough BQ studio, the gate turns on(30ms) / off(3000ms) in a cycle as if something was pulling it down when it goes high. When I refresh register in BQ studio, I see that DSG is LOW but that XDSG is LOW too.

Output of the BQ at DCHRG pin:

The problem happens when I'm over roughly 18V/18A WITH A LOAD. Below that or without load, the DSG pin act in normal operation.

Since the DCHRG pin of the BQ40Z80 is only connected to the gate of the Mosfet, I assume that there might be a config that is set wrong in my BQ programmation. Could you help me find what could be the problem?

Here is the schematic of the region of interest:

Thanks in advance,

Karl

  • 0
    TI__Mastermind 27321 points

    Hello Karl,

    You will probably need to add FETs in parallel to handle more current. The Q1 and Q3  FETs may not be rated for 18A, this can damage the FETs. Have you tried with a smaller load such as 2A? This will tell you if it is a FET issue.

    Adding FETs in parallel has its challenges as it will increase the gate capacitance, hence, it will slow down your switching. Also, when having multiple FETs, oscillations are seeing in the gate of the FETs, this can cause unintentional turn-OFFs/ONs. All of these concerns have been  addressed and compiled in multiple app notes. I will share them below.

    Note: This is for a different device, but same principle applies when using multiple FETs in parallel.

    1- Multiple FETs with the BQ76952, BQ76942 Battery Monitors
    2- How to debug FET failure?  Great one to read about!
    3- Parallel Paths with the BQ769x2 Battery Monitor

    Comments on your snipped shown above:
    1- We recommend to use 10Meg Ohms for the gate to source resistance. This will avoid any leakage current to the charge pump.
    2- The switching speed is limited with the gate resistance (R17 and R16). You may want to see if lowering helps with the issue, but most likely multiple FETs will be needed to support 18A load.

    Regards,
    Jose Couso

  • 0
    Prodigy 20 points

    Hi, after more debug, the answer was in the configuration memories.
    We designed the board to handle more than 50 Amps, so the FETs were not an issue.

    After changing the AOLD Register, the board can handle more current. Looks like the BQ40Z80 still apply some protection even in "manual" test mode. 

    Possible values of this register can be viewed in BQ40Z80 TRM at page 297.

    Thank you for your time,

    Karl